The communication links between two central offices are known as trunks. When the central offices are in different cities, they are called intercity trunks; when one central office is in a large city and the other central office is in a relatively small remote community, they are called outstate trunks. Multipair cables are often the transmission medium used for intercity and outstate trunks to connect central offices located in different cities. Intercity and outstate cables are typically buried but may also be installed aerially or in ducts.
Today most intercity and outstate trunks utilize carrier systems although some still transmit at voice frequency. The T1 carrier system, which is described in the Bell Laboratories Record, Vol. 40, No. 10, November 1962, pp. 358-363, is the dominant carrier system used to connect cities 10 to 50 miles apart. Voice frequency transmission is still important on these cables because some pairs are needed for: (1) voice frequency trunks (2) fault locate and order wire circuits for T1 carrier and (3) subscriber services in certain situations.
The T1 carrier system was designed to utilize a 22-gauge wood pulp insulated conductor exchange cable. This cable, which was originally developed for voice frequency transmission, has conductor pairs with a mutual capacitance of 83 nanofarads/mile (nF/mile). Each T1 system provides 24 channels on two pairs by displacing two voice channels. The repeater or regenerator equipment for carrier or high frequency transmission for the pulp cable was designed to use a repeater spacing which coincided with a standard load coil spacing of 6000 feet for voice frequency transmission. This allowed for easy transition from voice to carrier transmission. The repeater equipment was, and still is, designed to accommodate at 5.1 dB/kft loss, the loss of the 22-gauge pulp cable at T1 carrier transmission.
For several years after the introduction of the T1 carrier system, trunk expansion was accomplished by converting from voice to carrier transmission on up to about half the pairs in pulp cable. But system needs increased so that, beginning about 1972, cables frequently came to be needed in which all pairs could be used for carrier while still having voice transmitting capability.
Meanwhile, voice frequency cables had evolved to new designs which utilized new materials and manufacturing processes. For instance, pulp cable, which is susceptible to lightning damage in aerial installations, had been largely replaced for such applications by air core plastic insulated conductor (PIC) cable. As another example, pulp cable's susceptibility to failure by water entry in buried installations caused it to be supplanted by waterproof cable. Waterproof cable was originally made with solid plastic insulated conductors and the spaces between pairs filled with petroleum jelly. Now, waterproof cable is also made with dual expanded plastic insulated conductor (DEPIC) cable filled with petroleum jelly.
These replacement cables are all designed primarily for voice frequency transmission and accordingly, retain the 83 nF/mile standard mutual capacitance. Each replacement cable is also available in 19-, 22-, 24-, or 26-gauge conductor sizes, the same gauges as are standard for pulp cable. So long as the conductor material and gauge are duplicated and the mutual capacitance is 83 nF/mile, the voice frequency transmission of the pulp cable is duplicated. Hence the load coil spacings for the replacement cables are still normally 6000 feet and utilize the standard voice frequency electronics.
But to achieve the same voice frequency characteristics, the dielectric layers of the conductors in the replacement cables differ in dimension so that the three replacement cables have different transmission performances, i.e. different transmission loss at the T1 carrier frequency. For instance, for the 22-gauge air core PIC cable, the resultant T1 carrier repeater spacing is approximately 7.0 kft; for the filled (waterproof) PIC cable, it is 8.0 kft; and, for the filled DEPIC cable, it is 7.3 kft. From this it can be seen that the coincident load coil and repeater spacings conceived for the original 22-gauge pulp cable has been lost. The result often has been that the telephone companies space T1 carrier repeaters, even on these improved cables, at 6000 feet to be coincident with the load coils.
Disclosed in U.S. Pat. No. 4,058,669, issued to Nutt et al and assigned to the assignee of the present application, is an optimized T1 carrier and voice frequency cable. This cable has a coincident repeater and load coil spacing of 6000 ft (6300 ft maximum) and utilizes the newly available materials and processes. This cable is intended for metropolitan area trunks ranging in length from 2 to 20 miles with the cable usually installed in ducts and frequently on routes in which T1 repeater manholes are already built at spacings of 6000 ft. The design of this cable called for new voice frequency electronics, as well as new T1 carrier electronics. The new T1 carrier electronics has the same gain as the T1 repeater equipment designed for the 22-gauge pulp cable at the T1 design frequency.
More background information is given in applicants' article, "Multipair Cables for Digital Transmission," National Telecommunications Conference Proceedings, Dec. 3-6, 1978. This article, to the extent relevant to this application, is hereby incorporated by reference.
One object of the present invention is to design for transmission between cities a more efficient family of air core and waterproof cables with conductors having improved performance characteristics for both voice frequency and carrier frequency transmission. Another object is that such cables are compatible with the existing voice frequency electronics for 83 nF/mile cables and the existing carrier frequency electronics used for the cable disclosed in U.S. Pat. No. 4,058,669. Still another object is that such cables are cost efficient from both the carrier and voice frequency standpoints. Also, a still further object is that such cables use less copper as well as expanded insulation which is economical and efficient.